I don't think there are any
good free software Monopoly games out there, and I cannot really recommend any.
Yes, GtkAtlantic came to my mind, too. Works online, not sure about Windows.
But it's really not good. But to be fair, it still has a leading zero in the version number, this probably means it's not considered “finished”.
Right now (version 0.6.1), I think the graphics are just terrible, just boring naked GTK, no theming at all, not even the properties are properly named on the board.
Sure, it's just a board game, but at least some basic styling and work on look and feel should be done. It shouldn't be too hard, we're only talking about 2D graphics, here. It should look like a proper game, not like a boring office application.
The game manual is also crap. Sure, it's basically just Monopoly, but the devil lies in the details with all the special rules. Not having a rulebook for a board game implementation is a major fail IMO.
GtkAtlantic seems to implement the game itself as you would expect it, just the look and feel needs more work, lots of work.